Safety load and temperature control system for microwave ovens

ABSTRACT

A safety load and temperature control system for microwave ovens wherein a variable volume of water is passed through the oven for controlling the amount of energy received by a work piece therein. Microwave energy from magnetron tubes is introduced into the oven chamber for processing a work piece. The variable supply of water within the chamber is increased for small parts being cured and decreased for large parts being cured. The water is passed through a dielectrically transparent vessel within the chamber and is heated by the microwave energy proportional to the heated work piece or part. The temperature of the water leaving the chamber is indicative of the rate of change of the work piece temperature. A temperature controller has upper and lower limits that increase or decrease the water in the chamber and passing therethrough to maintain the work piece heating rate within a desired range. The variable supply of water also serves as a dummy load to absorb excessive radiant energy, preventing destructive feedback to the magnetron tubes. Microwave filters at water inlet and outlet ports keep the high frequency energy within the chamber.

United States Patent McKague, Jr. et al.

[ 51 May 16, 1972 [72] lnventors: Elbert L. McKague, Jr., Fort Worth;

Walter C. Cooper, Jr., Hurst, both of Tex.

[73] Assignee: The United States of America as represented by theSecretary of the Army 22 Filed: Dec. 7, 1970 211 Appl.No.: 95,783

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. JaegerAttorneyCharles K, Wright, Jr., William G. Gapcynski, Lawrence A.Neureither, Leonard Flank, Jack W. Voigt and Harold W. Hilton [5 7]ABSTRACT A safety load and temperature control system for microwaveovens wherein a variable volume of water is passed through the oven forcontrolling the amount of energy received by a work piece therein.Microwave energy from magnetron tubes is introduced into the ovenchamber for processing a work piece. The variable supply of water withinthe chamber is in- [52] U.S. Cl ..219/l0.55 creased for small partsbeing Cured and decreased for large [5 l] f "Hosb 9/06 parts beingcured. The water is passed through a dielectrically [58] Fleld of Search..2l9/l0.55 transparent vessel within the chamber and i heated y themicrowave energy proportional to the heated work piece or part. Thetemperature of the water leaving the chamber is indicative of the rateof change of the work piece temperature. [56] References Cned Atemperature controller has upper and lower limits that in- UNITED STATESPATENTS crease or decrease the water in the chamber and passingtherethrough to maintain the work piece heating rate within a 3,365,562H1968 Jeppson ..2l9/ 10.55 desired range. The variable supply of wateralso serves as a 3,549,848 12/1970 Williams ..2l9/l0.55 dummy load toabsorb excessive radiant energy, preventing 2,398,606 4/1946 Wang..2l9/l0.55 X destructive feedback to the magnetron tubes. Microwavefil- 3,335,253 8/1967 .leppson et a]. ..2l9/l0.55 ters at water inletand outlet ports keep the high frequency 3,474,210 10/1969 Gilbermann etal. ..2l9/10.55 energy within the chamber.

13 Claims, 4 Drawing Figures l2 C: 24 v U J O OLUME V 22 2? r TEMP FLOWIni| Il ill I ll N H H Y I I I V I H 11 w Patented May 16, 1-9723,663,783

2 Sheets-Sheet 1 VOLUME VOLUME CONTROL TEMP. FLOW Elbert L. McKogue, Jr.Walter C. Cooper, Jr.

INVENTORS,

Patented. 'May 16, 1912 3,663,783

:2 Sheets-Sheet 2 FIG. 4

INLET EXIT WATER 5 4, WATER RESERVOIR RESERVOIR FIG.3

Walter C. Cooper, r.

INVENTOR3.

Elbert L. McKogue Jr.

SAFETY LOAD AND TEMPERATURE CONTROL SYSTEM FOR MICROWAVE OVENSBACKGROUND OF THE INVENTION Microwave energy is useful in commercial andindustrial applications where it is desired to rapidly cure, sterilize,or dry a particular work piece or partfUses for microwave heatinginclude rapid preparation of frozen foods, sterilization of tools andfoods, curing. of plastics and ceramics and flame spectroscopy. In anassembly line operation, parts can automatically be carried through amicrowave heating chamber for processing. In microwave radiation forheating, the high frequency electric field energy changes to heatthrough the dielectric loss of the work piece. The heat generated in awork piece increases or decreasesin proportion to the microwavefrequency and the square of the electric field strength.

At higher microwave frequencies, the magnetron tube is employed as ahigh frequency generator for microwave ovens. To prevent damage of themagnetron tube, high levels of reflected energy must be avoided.Existing ovens rely on the operator for assurance that a power absorbingload adequate to prevent magnetron tube damage is in the oven duringoperation. This power load is normally the work piece itself. When awork piece of small size is placed in a microwave oven, it may absorbonly a small amount of the output energy, allowing feedback to occurwhich damages the magnetron tubes. Safety loads of stagnant water orresinous materials have been used to supplement a small work piece load.However, stagnant water rapidly boils away and resinous materials, oftenheated to deterioration, produce undesirable vapors. Further, aparticular oven with a set output of energy produces differencttemperature rise rates in difierent work parts, depending on the partweight and other factors.

SUMMARY OF THE INVENTION This invention relates to a microwave ovenhaving a variable and automatic safety load and temperature controlsystem.

The time required to cure a work part in a microwave oven is controlledby controlling the temperature rise rate within the oven. In productionoperation, for a known weight and specific heat of the material beingcured, the temperature control can be automated by setting machinelimits for temperatures which are predetermined from oven calibration.Electronic circuitry activates magnetron tubes that inject microwavefrequency energy into an inclosed metallic cabinet.-Microwave radiationis contained within the metallic chamber with the chamber load or workpiece absorbing the energy therin. A safety load of flowing water has avarying volume passing through the chamber which controls the amount ofenergy absorbed by the work piece and hence the temperature rise ratetherof. The water flows through a dielectrically transparent vesselwithin the microwave oven. Flowmeter valves adjust the amount of waterinside the oven in response to water temperature sensors.

An object of the present invention is to provide a variable dummy loadfor a microwave oven, allowing various sizes and materials to beprocessed through the oven chamber while protecting the power equipment.

Another object of the present invention is to provide a controlledenergy density within a microwave oven for controlling the time of cureof a part therein.

Still another object of the present invention is to provide atemperature control for a microwave oven that is also a safety load forthe oven.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view,partially in section, of a preferred embodiment of the invention.

F IG. 2 is a sectional view taken along the lines 22 of FIG. 1.

FIG. 3 is an elevational view of a microwave oven with inlet and outletreservoirs of water to prevent energy leakage.

FIG. 4 is an elevational view of an alternative volume control forvarying the water level within the chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENT Continuous safe operation ofmicrowave ovens requires a power absorbing material inside the ovencabinet to prevent build up of excessive concentrations of energy. Withinadequate power absorbing material in an oven, high energy standingwaves are formed and microwave energy is fed back into the magnetrontubes, shorting them out. Standing waves may also form in such a way asto cause excessive heat development in the magnetron tube, resulting inrapid tube damage.

When a microwave oven is to be operated for a long period of time (10minutes or more), it is not practical to use a solid stationary materialto absorb power, because the increasing temperatures degrade thematerial. Plastic power absorbers liberate offensive fumes astemperatures rise. A flowing liquid such as water that absorbs powerprovides a capability for curing parts with uninterrupted, nonhazardousoperation. Referring now to the drawings wherein like numbers representlike parts in the figures, FIG. l,discloses apparatus representing afirst embodiment of a microwave oven having a safety load and controlsystem. A microwave oven 10 has a metal microwave chamber 12 therein anda control panel 14 adjacent a side of the oven. A plurality of magnetrontubes 16 direct microwave energy from the chamber ceiling toward a workpiece 18 supported within the chamber. A liquid such as water isintroduced into chamber 12 at a known temperature through a metalflow-meter-controlled pipe 22, which is attached to the chamber wall ofthe oven. A control valve 24 in pipe 22 is connected to control panel 14and may be operated conventionally as by an electrical or pressure servosystem to control the volume of water passing therethrough. A fluidvessel 26 within chamber 12 is connected at one end to an input pipe 27and at another end to an outlet pipe 28. The inlet pipe, outlet pipe andfluid vessel, which may be glass, are dielectrically transparent to themicrowave energy introduced into the chamber. Inlet pipe 27 is connectedto metal pipe 22 for passing a large volume of water through the pipesand into vessel 26. Similarly a metal drainage pipe 30 is connected tothe chamber wall of oven 10 and is joined by conventional methods toglass outlet pipe 28 for passing water from vessel 26 to the outside ofchamber 12. Water is flowed through the glass pipe and vessel,-beingheated by the microwaves during operation and thereby serving as asafety or dummy load for the oven. Microwave energy is absorbed andconverted to heat by the work piece 18. and by the dummy load, the waterwithin vessel 26. Microwave energy may be repeatedly reflected in thechamber with waves passing through the load releasing energy as'heat tocure the work piece. The dummy load absorbs excessive relfections andradiation reducing damaging feedback of energy to the magnetrons.

Due to the high attenuation factor in water there is little if anytendency for pipes 22 and 30 to act as a waveguide for the microwaveenergy. However, grounding pins 32 are incor porated in metal pipes 22and 30 at periodic intervals to further protect against any tendency ofthe pipes to produce a waveguide efiect. A flow meter valve 34 on outletpipe 30, when operated in conjunction with inlet valve 24, adjusts andcontrols the amount of water inside the oven. The amount of water invessel 26 is increased by opening inlet flow valve 24 more than exitflow valve 34 is opened. Both valves are partially open at all timesallowing continuous flow of water through the oven to preventdevelopment of boiling temperature in the water.

A thermocouple probe 40, temperatue sensor, is inserted into pipe30 tomeasure the temperature of the stream of water exiting oven 10 forproviding an indication of both the power absorbed by the safety waterload and the power absorbed by the part. A signal from thermocouple 40activates a servocontrol within control panel 14 to increase the volumeof water in the oven by increasing the opening of valve 24 if the exitwater temperature exceeds a preselected value. When exit watertemperature exceeds the preselected value, the heating rate of the workpiece is excessive. With the volume of water in vessel 26 increased theoutflow temperature begins to drop and the volume of water adjusts asthe servo system seeks to maintain the preselected temperature of theexiting water.

Similarly, the volume of water in vessel 26 is decreased if thetemperature of exiting water falls below a preselected value, causingthe thermocouple to actuate a servo-control for outlet flow-meter valve34. This thermocouple control operates similarly to a thermostat andvarious flow control methods may be employed in accordance with the loadcharacteristics. The initial volume of water supplied through valve 24is preselected in accordance with a desired cure time and temperaturefor the work piece. The volume of water required in vessel 26 can becontrolled by a float meter within the vessel or by establishing aselected differential between inlet and outlet water flow for a specificperiod of time, followed by equalization of the flow rates. A continuousdrain line 36 may be connected around valve 34 to allow a predeterminedminimum water flow through pipes 30, which also allows valve 34 tofunction as a fine adjustment in temperature control.

The time required to cure a work piece such as a plastic part in oven 10is controlled by controlling the temperature rise rate. For a knownweight and specific heat of the material to be cured, the oven limitsfor temperature are calibrated and set, allowing automatic control ofthe part being cured. The power absorbing safety load of water insidechamber 12 prevents buildup of excessive concentrations of microwaveenergy. Without adequate power absorbing material in the oven, highenergy standing waves are formed and microwave energy is fed back intothe magnetron tubes, resulting in rapid tube damage. Thus a microwaveoven safety load is provided that simultaneously provides control of thework piece temperature rise rate.

As noted in FIGS. 1 and 2, the inner oven wall in the region inclosed bythe inside diameter (ID) of pipes 22 and 30 has a plurality of verysmall diameter holes 44. Holes 44 form a perforated screen 46 throughwhich a large volume of water can flow. Screens 46 are electricallygrounded, being part of the oven closure and the microwave energy isreflected thereby. Each hole 44 is approximately 0.010 inches indiameter to prevent leakage of the energy through the water filled hole.Glass pipes 27 and 28 inclose the areas of perforation inside the ovenwall, guiding the flow of water therethrough. Forming a part of the ovenwall, screens 46 prevent the microwave energy from escaping the chamberand can be used instead of using grounding pins 32 or in additionthereto.

The effective diameter of a hole filled with a dielectric material isenlarged with respect to passage of microwave energy therethrough. Theeffective diameter of pipes 22 and 30 is enlarged by the factor of thesquare root of the dielectric coefficient of the material (water) at theintended frequency of the energy. The dielectric coefficient of water isabout 80 and the effective diameter of a hole through which water mightflow is WE or approximately 9 times the actual diameter. To preventleakage of energy the hole must be less than one quarter wave length ineffective size. The actual possible diameter of holes 44 is For F, 2,450Megacycles, AA 1.20 inches and the maximum possible diameter isapproximately (1.20 in.)/9 0.133 in., or slightly larger than it inch.Thus 0.010 inch is well below the maximum diameter that prevents leakageof energy through the holes 44.

If inlet pipe 22 is completely sealed and grounded to the oven wall, thediameter of the inlet pipe need not be restricted when the pipe leadsinto a metal-inclosed water reservoir large enough to attenuate orabsorb all energy that escapes into the pipe. FIG. 3 discloses a metalinlet water reservoir 51 and outlet reservoir 52 having a plurality ofinput and output metal pipes 54 connected to microwave oven 10. Aplurality of glass pipes 56 within the oven chamber 12 connect torespective input and output pipes 54 and pass water therethrough toabsorb the excessive radiant energy. Several diameters of glass pipesallow large variations in the quantity of water passing through thecuring chamber, providing load and temperature control by filing onlythe number of glass tubes needed to control the temperature for theparticular size of work piece.

The servo-controlled flow meter valves can be eliminated by using aconstant rate of water flow into the oven. Valve 24 is manually set toallow the constant rate of water flow, filling dielectricallytransparent vessel 26 to some level. Water is gravity drained fromvessel 26 through outlet pipes 28 and 30 that are large enough indiameter to assure a draining rate capability greater than the fillingrate. As shown in FIG. 4, a transparent flexible hose or tube 60 isconnected to the drain end of metal pipe 30 outside the oven wall. Pipe30 extends from the oven a distance great enough to prevent radiation ofmicrowave energy therefrom. Drain line 36 provides a constant tricklepath around hose 60, insuring a minimum drainage rate for vessel 26. Atransparent stand pipe 62 is connected in hose 60 near the junction ofpipe 30 and the hose. Stand pipe 62 is taller than the height of thevessel in the oven and is at the apex of the flexible hose to preventsiphoning of the vessel. The height of the apex of hose 60 is changed topromote or restrict gravity induced flow of water from vessel 26.Thermocouple 40 senses the outlet water temperature and passes an outputsignal to control panel 14 for raising or lowering the flexible hose asneeded to maintain a specific temperature in vessel 26. Hose 60 israised or lowered by motor driven elevator screw 64 and is adjacent afixed backboard or support post 66, having volume graduations thereonfor indicating the volume of water in the chamber. Stand pipe 62 can beused to guide the manual presetting of initial water volume. Upper andlower safety switches 68 on support post 66 prevent screw 64 from beingdriven beyond safety limits.

Thus a safety load and temperature control system is disclosed formicrowave ovens wherein water is passed through an oven chamber andoutlet water temperature is monitored. The monitored outlet temperatureis used to control water volume within the chamber. An upper limitcontrol on the temperature controlling device prevents equipment damageand varying the volume of water in the chamber and the time thereinprovides various heating rates for work pieces.

At the lower safety switch limit maximum water is flowing through drainhose 60, rapidly replacing the volume of water in vessel 26 for maximumheat transfer from the system. At the upper safety switch limit, waterflow is only through drain line 36, restricting gravity induced flow ofwater from the vessel.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. For example radiation from theinlet and outlet metal pipes can be prevented by crumpled metal foilaround the pipes or by one or more bends in the pipe at pointscorresponding to halfwave lengths of the radiant energy. The stand pipecan be transparent, graduated and employed with a non-transparent hose.It is therefore to be understood that within the scope of the apendedclaims the invention may be practiced otherwise than as specificallydescribed herein.

We claim:

1. A safety load and temperature control system for microwave ovens usedin curing various sizes of work pieces, comprising: a microwave chamberhousing having metal chamber walls for reflecting microwave energywithin said chamber, a microwave emitting source within said chamber,first and second opeings through opposing chamber walls, first andsecond liquid conducting metal pipes external to said oven and havingrespective first ends thereof inserted through said openings and sealedto the inner wall of said chamber for coupling a variable volume ofliquid into said chamber, means communicating with said first and secondmetal pipes for conveying said volume of liquid through said chamber forproviding a variable safety and dummy load and temperature control forsaid chamber, control means for varying said volume of liquid conveyedto and from said chamber and heat responsive means for measuring thetemperature of said liquid leaving said chamber.

2. A safety load control system for microwave ovens as set forth inclaim 1 wherein said conveying means is a microwave frequencydielectrically transparent vessel within said chamber havingdielectrically transparent input and output tubes, said input tube beingjoined to said first chamber wall opening and pipe therethrough forconveying said liquid into said vessel, said output tube being connectedto drain said vessel and joined to said second opening and metal pipefor conveying said liquid out of said vessel and chamber.

3. A safety load control system as set forth in claim 2 wherein saidheat responsive means is a thennocouple in said second metal drain pipeexternal to said microwave oven.

4. A safety load control system as set forth in claim 3 wherein saidcontrol means comprises an inlet control valve in said first metal pipeand an outlet control valve in said second metal pipe for increasing anddecreasing the liquid volume level within said vessel for controllingthe temperature within the chamber.

5. A safety load control system as set forth in claim 4 wherein saidcontrol means further comprises a control panel having a servo systemcontrol with an input connected to an output of said thermocouple forresponding to said liquid temperature changes, said servo system havingfirst and second outputs coupled to said input and output valvesrespectively for automatically operating said control valves.

6. A safety load control system as set forth in claim 5 wherein saidinlet and outlet metal pipes are grounded to said microwave oven toprevent microwave energy from escaping said chamber.

7. A safety load control system as set forth in claim 5 and furthercomprising a metal screen forming the ends of said first and secondmetal pipes, said screens having a plurality of holes therein, saidholes having a diameter less than a quarter of a wave length of themicrowave energy from said source, and wherein said liquid is water andsaid vessel and tubes are glass.

8. A safety load control system as set forth in claim 1 wherein saidliquid is water and further comprising a plurality of dielectrictransparent vessels within said chamber, a plurality of input and outputmetal pipes communicating into said chamber for conveying water into andout of respective said vessels, metal inlet and exit water reservoirsadjacent said oven and connected to said metal pipes for conveying waterto and from said microwave chamber while absorbing energy leakage fromsaid chamber, and said vessels being tubular glass of unequal diameter.

9. A safety load control system as set forth in claim 3 and furthercomprising a flexible hose connected to the other or drain end of saidsecond metal pipe for draining said vessel, a motor driven elevatorscrew connected for raising and lowering said hose to restrict orpromote liquid flow therethrough, a support post for said elevator screwand a transparent stand pipe connected in said hose at the apex of thehose for indicating the volume of water within said chamber, and saidcontrol means being responsive to an output signal of said thermocouplefor activating said motor driven screw.

10. A safety load control system as set forth in claim 9 wherein saidliquid is water and said vessel and tubes are glass, said outlet tubeand metal pipe having a diameter that provides a drainage rate greaterthan the input filling rate of the vessel for gravity drainage thereof.

11. The method of curing various sizes of work pieces within a microwaveoven that provides a variable safety load and temperature control systemfor the oven, said oven including a curing chamber and a dielectricallytransparent vessel within said chamber, comprising the steps of:

placing a work piece to be cured within said oven chamber,

passing a liquid through said vessel for providing a dummy or safetyload within said chamber, radiating microwave energy into said chamberfor curing said work piece,

measuring the temperature of liquid exiting said chamber,

and

adjusting the volume of said liquid passing through the chamber inresponse to variations in the measured temperature for providing asafety load that absorbs excessive radiant energy and controls the rateof heating of said work piece within said chamber.

12. The method of heating work pieces within a microwave oven thatprovides a variable safety load and temperature control as set forth inclaim 1 1, comprising the steps of:

increasing and decreasing the volume of said liquid passing through thechamber in response to respective increases and decreases of themeasured temperature, and

passing water through said vessel for providing said safety load.

13. The method of heating work pieces within a microwave oven thatprovides a variable safety load and temperature control as set forth inclaim 12, comprising the steps of:

injecting water into said vessel for providing said safety load, gravitydraining said water from said vessel and chamber, restricting andpromoting water flow from said vessel by elevating or lowering thegravity flow level from said chamber in response to said measuredtemperature variations.

1. A safety load and temperature control system for microwave ovens usedin curing various sizes of work pieces, comprising: a microwave chamberhousing having metal chamber walls for reflecting microwave energywithin said chamber, a microwave emitting source within said chamber,first and second opeings through opposing chamber walls, first andsecond liquid conducting metal pipes external to said oven and havingrespective first ends thereof inserted through said openings and sealedto the inner wall of said chamber for coupling a variable volume ofliquid into said chamber, means communicating with said first and secondmetal pipes for conveying said volume of liquid through said chamber forproviding a variable safety and dummy load and temperature control forsaid chamber, control means for varying said volume of liquid conveyedto and from said chamber and heat responsive means for measuring thetemperature of said liquid leaving said chamber.
 2. A safety loadcontrol system for microwave ovens as set forth in claim 1 wherein saidconveying means is a microwave frequency dielectrically transparentvessel within said chamber having dielectrically transparent input andoutput tubes, said input tube being joined to said first chamber wallopening and pipe therethrough for conveying said liquid into saidvessel, said output tube being connected to drain said vessel and joinedto said second opening and metal pipe for conveying said liquid out ofsaid vessel and chamber.
 3. A safety load control system as set forth inclaim 2 wherein said heat responsive means is a thermocouple in saidsecond metal drain pipe external to said microwave oven.
 4. A safetyload control system as set forth in claim 3 wherein said control meanscomprises an inlet control valve in said first metal pipe and an outletcontrol valve in said second metal pipe for increasing and decreasingthe liquid volume level within said vessel for controlling thetemperature within the chamber.
 5. A safety load control system as setforth in claim 4 wherein said control means further comprises a controlpanel having a servo system control with an input connected to an outputof said thermocouple for responding to said liquid temperature changes,said servo syStem having first and second outputs coupled to said inputand output valves respectively for automatically operating said controlvalves.
 6. A safety load control system as set forth in claim 5 whereinsaid inlet and outlet metal pipes are grounded to said microwave oven toprevent microwave energy from escaping said chamber.
 7. A safety loadcontrol system as set forth in claim 5 and further comprising a metalscreen forming the ends of said first and second metal pipes, saidscreens having a plurality of holes therein, said holes having adiameter less than a quarter of a wave length of the microwave energyfrom said source, and wherein said liquid is water and said vessel andtubes are glass.
 8. A safety load control system as set forth in claim 1wherein said liquid is water and further comprising a plurality ofdielectric transparent vessels within said chamber, a plurality of inputand output metal pipes communicating into said chamber for conveyingwater into and out of respective said vessels, metal inlet and exitwater reservoirs adjacent said oven and connected to said metal pipesfor conveying water to and from said microwave chamber while absorbingenergy leakage from said chamber, and said vessels being tubular glassof unequal diameter.
 9. A safety load control system as set forth inclaim 3 and further comprising a flexible hose connected to the other ordrain end of said second metal pipe for draining said vessel, a motordriven elevator screw connected for raising and lowering said hose torestrict or promote liquid flow therethrough, a support post for saidelevator screw and a transparent stand pipe connected in said hose atthe apex of the hose for indicating the volume of water within saidchamber, and said control means being responsive to an output signal ofsaid thermocouple for activating said motor driven screw.
 10. A safetyload control system as set forth in claim 9 wherein said liquid is waterand said vessel and tubes are glass, said outlet tube and metal pipehaving a diameter that provides a drainage rate greater than the inputfilling rate of the vessel for gravity drainage thereof.
 11. The methodof curing various sizes of work pieces within a microwave oven thatprovides a variable safety load and temperature control system for theoven, said oven including a curing chamber and a dielectricallytransparent vessel within said chamber, comprising the steps of: placinga work piece to be cured within said oven chamber, passing a liquidthrough said vessel for providing a dummy or safety load within saidchamber, radiating microwave energy into said chamber for curing saidwork piece, measuring the temperature of liquid exiting said chamber,and adjusting the volume of said liquid passing through the chamber inresponse to variations in the measured temperature for providing asafety load that absorbs excessive radiant energy and controls the rateof heating of said work piece within said chamber.
 12. The method ofheating work pieces within a microwave oven that provides a variablesafety load and temperature control as set forth in claim 11, comprisingthe steps of: increasing and decreasing the volume of said liquidpassing through the chamber in response to respective increases anddecreases of the measured temperature, and passing water through saidvessel for providing said safety load.
 13. The method of heating workpieces within a microwave oven that provides a variable safety load andtemperature control as set forth in claim 12, comprising the steps of:injecting water into said vessel for providing said safety load, gravitydraining said water from said vessel and chamber, restricting andpromoting water flow from said vessel by elevating or lowering thegravity flow level from said chamber in response to said measuredtemperature variations.